1. Technical Field of the Invention
The invention relates to a method for determining the anatomical dead space in the respiratory tract of living organisms by the continuous and simultaneous measurement of flow (F) and respiratory air density (D) during exhalation (EX) over the time (T).
2. Description of the Prior Art
The respiratory tract includes the nose, pharynx, larynx, trachea, bronchia, bronchioli and alveoli. The trachea branches shortly before entering the lung into the left and right bronchi, also known as the bronchus principalis.
The lung is part of the respiratory tract and consists of air-carrying ducts, the bronchial system, which guides ambient air into the alveoli, in which gas is exchanged with the blood, that is to say oxygen is removed from the ambient air and carbon dioxide is added. The alveoli consist of very numerous and very small sacs, whose diameter is less than a millimeter. In an adult human, the internal surface area of all the alveoli is estimated at about 100 square meters.
The cavities of the respiratory tract, which guide the air from the mouth to the alveoli, are known as the “anatomical dead space”, since they are not involved in gas exchange. Instead, they are only used for cleaning, preheating and moistening the respiratory air.
Since the lung unfortunately often has to battle with illnesses and/or can be pathologically changed, but a direct diagnosis of the lung itself is not possible, since it is not visible, imaging methods like X-ray equipment, magnetic resonance tomography (MRT) and ultrasound equipment are used to make fundamental statements about the change of the lungs. However, this method only permits qualitative diagnoses about changes of the lungs. Another problem is that the human respiration must not be interrupted during the diagnosis, and ideally should not be restricted at all.
The most diverse measuring instruments are therefore known in the prior art for quantitative determination of the lung function as a diagnosis aid for a multiplicity of lung diseases. Such instruments more or less accurately measure the flow of respiratory air, that is to say the air mass, and the respiratory air density during inhalation and exhalation.
Some known measurement instruments require the patient to exhale completely, inhale completely and/or not to breathe at all for a short time. Experience has shown that this breathing regime is not only experienced as unpleasant and inconvenient, but is also very often followed incorrectly, irregularly or with delay, as a consequence of which serious measurement errors can occur, which greatly restrict the diagnostic benefit of these devices.
Another disadvantage of many known methods is that the CO2 concentration in the air is measured via complicated mass spectrometers or relatively slow IR analysers. This equipment is mostly very complex to manufacture and operate, and generally requires the cooperation of the patient. It can therefore not be used for small children, multimorbid and/or severely ill patients, and/or those with dementia. Another disadvantage is the very high costs and the large volume of the entire arrangement, including all ancillary equipment.
The common factor to all the devices is that they are not capable of distinguishing between the “dead space air” from the dead space of the lung system and the “alveolar air” from the air sacs—the alveoli—actively involved in gas exchange. The anatomic dead space consists of the mouth and pharynx space, the contiguous airways, the trachea and, within the lung, of the bronchia, which extend from the windpipes into both lungs, and there continually branch until they reach the lung sacs.
According to the unanimous opinion of persons skilled in the art, the dead spaces do not take part in the gas exchange of the respiratory air into the blood at all, or only to a marginal extent. The function of the dead spaces is to clean, thermally condition (warm) the respiratory air and saturate the inhaled air to 100% relative humidity. Therefore, for precise determination of the air that has actually entered the alveoli, the alveolar air, the dead space volume of the entire respiratory volume can also be deducted.
In the prior art, the start of exhalation can be measured with good accuracy. However the transition from the dead space air expelled during exhalation to the alveolar air, i.e. the air volume coming back from the lung sacs, cannot be measured with the desirable accuracy. All the equipment and described principles of the prior art are too slow, too inaccurate, or both.
The German Offenlegungsschrift DE 1 918 566, Erich Jäger, of Apr. 11, 1989, describes a “device for examining lung function”, in which, before the actual measurement, a “magnetic valve” opens, so that the “portion of the expiratory air corresponding to the dead space” flows out through the outlet. Therefore, very generally, a particular portion of the exhaled air is first classified as dead space air.
This device therefore refrains in advance from accurate measurement of the dead space volume, but is restricted to expelling a portion of the exhaled air unmeasured into the open that is so large that the measurement only takes place with alveolar air. However, this principle requires that only particular conditions and compositions in the air can be measured, but not the volume thereof.
A ratio between the dead space and alveolar space that is distorted for pathological reasons can therefore only be measured indirectly and hence only with very restricted accuracy with such an instrument.
German Offenlegungsschrift DE 28 12 379, Udo Smidt, discloses a device for lung diagnostics, in particular for diagnostic of lung emphysema, which values the greatly increased portion of the mixed air volume, compared to healthy persons, as an indicator. To this end, a gas analyser is preferably used, the CO2 measurement sensor of which operates according to the infrared absorption principle. In addition, an inhalation stream receptor is necessary to determine the respiration volume. From the variation of CO2 concentration with time and the respiration volume with time, the range of constant increase of CO2 concentration after exhalation of the dead space air can be used for calculating the mixed air volume. The increase of the mixed air volume in ratio to the total volume is evaluated as an indicator of the severity of the lung emphysema.
The method could not become established at the time of its disclosure because of the, at that time, impracticably high effort for performing the computational functions, that is to say in the absence of the microprocessors that are available in the current state of the art. Another disadvantage is the complicated infrared absorption measurement sensor that was preferred at that time.
With modern microprocessor technology and better sensors, this method could be realized in the current state of the art. But, even then, there remains the disadvantage that the measurement of the actual, that is to say the functional, dead space volume is not possible, but can only be estimated indirectly with a very high tolerance.
In the absence of measurement methods, which are mostly too slow, it is hardly known that the transition from the dead space air to the alveolar air is marked by the dramatic decrease of the curve of respiratory air density over time to a value that is constant for a short time. That may be one reason why until now no measurement method has been known that permits this point to be determined with the desirable accuracy.
For example, the curve of CO2 concentration with time, published in DE 28 12 379 under FIG. 2 does not show this decrease at all.
The portion of the expiration volume can be calculated using Bohr's dead space formula. With the following abbreviations    Vat=Anatomic dead space volume    Vg=Breath volume    Di=CO2 concentration of fresh air    De=CO2 concentration of expirate    Da=CO2 concentration of alveolar air
The following sequences and relationships apply:
Before inspiration, the alveolar space and dead space are still filled with alveolar gas. After the Inspiration of the breath volume Vg, the alveolar space has expanded by Vat, but only the portion Vg-Vat reaches the alveolar space, while the rest remains in the dead space. The portion in the alveolar space mixes with the alveolar gas, so that the latter is refreshed. On expiration, first fresh air is exhaled from the dead space, then alveolar gas; the gas concentration then again reaches the same value as before inspiration. For the relationship between the individual parameters, the Bohr's formula applies:
                    Vat        ·        Di            +                        (                      Vg            -            Vat                    )                ·        Da              =          Vg      ·      De            Vat    =          Vg      ·                        Da          -          De                Da            
A diagnosis according to this principle first requires the correct assignment to the areas of fresh air (Di), expirate (De) and alveolar air (Da) and, within this area, correct measurement of the CO2 concentration in each case. According to Bohr's formula, the dead space can then be calculated. The result, however, is only accurate enough to maintain the proportionality of the respective CO2 concentrations, which, however, can be changed by other effects, in particular in the case of a diseased lung. Based on Bohr's formula, therefore, only a trend, not an accurate diagnosis is possible.